Description Introduction. The Atacama Desert is considered the driest (<2mm year-1 rainfall) and the oldest (90-150 My) desert on Earth. Its hyperarid core is one of the most extreme environments and has been considered lifeless. In this area atmospheric precipitations are extremely scarce and irregular which together with high rates of evaporation (1300-1600 mm year-1) generates long periods of desiccation. Aside from the hyperaridity, Atacama Desert also has other environmental factors that impose severe limits to life, such as elevated UV-radiation levels and intense solar radiation, large daily fluctuations of temperature and relative humidity, extreme oligotrophic conditions, and often elevated concentrations of inorganic oxidants and salts in soils. In this scenario, as in others deserts, lithic ecosystems become the dominant biological components. Among all the lithobiontic habitats discovered forming “islands” or “oasis” across the Atacama Desert, the endolithic colonization of superficial halite nodules that are present in different fossil salt flats scattered in the hyperarid core is particularly relevant. These nodules are unique formations of the Atacama Desert which allow the establishment of microbial communities based on photosynthesis in areas previously described as "dry limit for photosynthetic life on Earth".
[EN] Objectives. The main objective of this PhD thesis is to carry out a detailed characterization of endolithic microbial communities in superficial halite nodules providing a description of their ecology under three different but complementary methodological approaches which define the partial objectives : 1) Characterization of abiotic elements that influence microbial colonization of halite nodules through a comparative analysis of : microclimatic conditions in the environment and inside halite nodules which are present in different locations in the hyperarid core of the Atacama Desert, its chemical and mineralogical composition and their porous structure. 2) Microhabitat characterization and analysis of microbial community organization inside nodules, morphological diversity and relations between microorganisms, using different microscopy techniques. 3) Microbial diversity characterization inside halite nodules using a comparative analysis of community structure in different locations of Atacama Desert by DGGE and an estimation of its biomass analysing the amount of ATP.